How are the sand and the ash separated in a Fluidized bed boiler?

Considering the bed material is sand, after the combustion the bed is a mix of sand and bottom ash. If the ash aculates, the level of the bed raises, so you "drain" some bed material and add new sand. This "drained" bed material is a mix of sand and ash, so how can you separate those so you can reuse the sand and dispose the ash? Thanks for your help.